This invention relates to fuel-cooled aircraft operational subsystems, but, more particularly, to a system which increases an aircraft's heat capacity by integrating airframe and engine thermal management systems, utilizing the aircraft's fuel as both coolant and heat sink.
The high specific heat and density of fuel, compared with that of ram air, makes fuel an attractive onboard heat sink and allows use of smaller heat exchangers in airframe subsystems. Moreover, fuel, in its closed containment system, is an inherently clean heat sink (respecting radiation, biological and chemical contamination) and entails no penalty of increased radar cross section, as do ram air systems. During aerodynamically "hot" flight conditions, fuel temperature is cooler than that of ram air and more compatible with subsystem temperature limits, making fuel more attractive as a heat sink than such ram air. In addition, it is a heat sink that is consumed through useful combustion in tne engine.
Although fuel has advantages over ram air as a heat sink, it has certain limitations. One of these is that the temperature of jet fuels must be kept below specified maximums at the engine combustor nozzles to prevent fuel coking, gumming, and varnishing. The maximum permissible temperature for JP-4 and JP-5 fuels is 325.degree. F. Also, as fuel temperatures and/or flight altitudes increase, normal fuel tank pressures can result in excessive boiloff. Prevention of boiloff by increasing tank pressure results in structural weight penalties.
Supersonic and hypersonic aircraft that are aerodynamically "hot" may use fuel to absorb frictional heat produced by the airstream passing over wing and fuselage surfaces. One prior art method of using fuel as such a heat sink makes use of a catalyst. The catalyst promotes an endothermic chemical reaction wnich denydronates the fuel, allowing it to absorb more heat. Such a metnod is disclosed in U.S. Pat. No. 3,438,602 to Noddings, et al. This type of system chemically breaks down the fuel into smaller molecules and alters the fuel's characteristics as a propellant. Such a system requires temperatures of 700 to 1000 degrees F. for operation and is consequently limited in application to high supersonic or hypersonic aircraft.
Other prior art systems for heat management in high speed aircraft incorporate fuel refrigeration systems. A major disadvantage of such systems is that additional power is required for the refrigeration itself, and this places a drain on energy producing systems of the aircraft. A further disadvantage of such systems is that they control the temperature of fuel at the engine inlet rather than at the engine fuel nozzles, where fuel temperatures are most critical. Such a system is disclosed in U.S. Pat. No. 4,505,124 to Mayer.
Other prior art aircraft heat management systems use fuel to cool aircraft subsystems as well as the airframe. An example of such is presented is U.S. Pat. No. 4,273,304 to Frosch, et al. The Frosch system, however, refrigerates the fuel, necessitating additional power for the refrigeration system, as with some of the other prior art systems discusse above. Another disadvantage of Frosch cooling is that it requires higher quantities of heat absorbing fuel as cooling demands of tne airframe and aircraft subsystem increase. Thus, near the end of a given mission, when fuel quantity is minimal, this system has a significantly reduced cooling capability.